Innovative 3-D designs can more than double solar power generated from a given area

March 26, 2012
by David L. Chandler

Two small-scale versions of three-dimensional photovoltaic arrays were among those tested by Jeffrey Grossman and his team on an MIT rooftop to measure their actual electrical output throughout the day. Photo: Allegra Boverman

(PhysOrg.com) -- Intensive research around the world has focused on improving the performance of solar photovoltaic cells and bringing down their cost. But very little attention has been paid to the best ways of arranging those cells, which are typically placed flat on a rooftop or other surface, or sometimes attached to motorized structures that keep the cells pointed toward the sun as it crosses the sky.

Now, a team of MIT researchers has come up with a very different approach: building cubes or towers that extend the solar cells upward in three-dimensional configurations. Amazingly, the results from the structures they've tested show power output ranging from double to more than 20 times that of fixed flat panels with the same base area.

The biggest boosts in power were seen in the situations where improvements are most needed: in locations far from the equator, in winter months and on cloudier days. The new findings, based on both computer modeling and outdoor testing of real modules, have been published in the journal Energy and Environmental Science.

"I think this concept could become an important part of the future of photovoltaics," says the paper's senior author, Jeffrey Grossman, the Carl Richard Soderberg Career Development Associate Professor of Power Engineering at MIT.

The MIT team initially used a computer algorithm to explore an enormous variety of possible configurations, and developed analytic software that can test any given configuration under a whole range of latitudes, seasons and weather. Then, to confirm their model's predictions, they built and tested three different arrangements of solar cells on the roof of an MIT laboratory building for several weeks.

While the cost of a given amount of energy generated by such 3-D modules exceeds that of ordinary flat panels, the expense is partially balanced by a much higher energy output for a given footprint, as well as much more uniform power output over the course of a day, over the seasons of the year, and in the face of blockage from clouds or shadows. These improvements make power output more predictable and uniform, which could make integration with the power grid easier than with conventional systems, the authors say.

The basic physical reason for the improvement in power output  and for the more uniform output over time  is that the 3-D structures' vertical surfaces can collect much more sunlight during mornings, evenings and winters, when the sun is closer to the horizon, says co-author Marco Bernardi, a graduate student in MIT's Department of Materials Science and Engineering (DMSE).

The time is ripe for such an innovation, Grossman adds, because solar cells have become less expensive than accompanying support structures, wiring and installation. As the cost of the cells themselves continues to decline more quickly than these other costs, they say, the advantages of 3-D systems will grow accordingly.

"Even 10 years ago, this idea wouldn't have been economically justified because the modules cost so much," Grossman says. But now, he adds, "the cost for silicon cells is a fraction of the total cost, a trend that will continue downward in the near future." Currently, up to 65 percent of the cost of photovoltaic (PV) energy is associated with installation, permission for use of land and other components besides the cells themselves.

Although computer modeling by Grossman and his colleagues showed that the biggest advantage would come from complex shapes  such as a cube where each face is dimpled inward  these would be difficult to manufacture, says co-author Nicola Ferralis, a research scientist in DMSE. The algorithms can also be used to optimize and simplify shapes with little loss of energy. It turns out the difference in power output between such optimized shapes and a simpler cube is only about 10 to 15 percent  a difference that is dwarfed by the greatly improved performance of 3-D shapes in general, he says. The team analyzed both simpler cubic and more complex accordion-like shapes in their rooftop experimental tests.

At first, the researchers were distressed when almost two weeks went by without a clear, sunny day for their tests. But then, looking at the data, they realized they had learned important lessons from the cloudy days, which showed a huge improvement in power output over conventional flat panels.

For an accordion-like tower  the tallest structure the team tested  the idea was to simulate a tower that "you could ship flat, and then could unfold at the site," Grossman says. Such a tower could be installed in a parking lot to provide a charging station for electric vehicles, he says.

So far, the team has modeled individual 3-D modules. A next step is to study a collection of such towers, accounting for the shadows that one tower would cast on others at different times of day. In general, 3-D shapes could have a big advantage in any location where space is limited, such as flat-rooftop installations or in urban environments, they say. Such shapes could also be used in larger-scale applications, such as solar farms, once shading effects between towers are carefully minimized.

A few other efforts  including even a middle-school science-fair project last year  have attempted 3-D arrangements of solar cells. But, Grossman says, "our study is different in nature, since it is the first to approach the problem with a systematic and predictive analysis."

David Gracias, an associate professor of chemical and biomolecular engineering at Johns Hopkins University who was not involved in this research, says that Grossman and his team have demonstrated theoretical and proof-of-concept evidence that 3-D photovoltaic elements could provide significant benefits in terms of capturing light at different angles. The challenge, however, is to mass produce these elements in a cost-effective manner.

Related Stories

Zhengrong Shi, an engineer turned entrepreneur, has become one of Chinas richest citizens by betting heavily on the future of solar power. In a talk on Tuesday at MIT, he outlined his vision for how he believes solar ...

If a new development from labs at MIT pans out as expected, someday the entire surface area of a buildings windows could be used to generate electricity  without interfering with the ability to see through them.

The biggest hurdle to widespread implementation of solar power is the fact that the sun doesn't shine constantly in any given place, so backup power systems are needed for nights and cloudy days. But a novel system designed ...

Jeffrey Grossman says Cambridge has a better climate than California  for carrying out materials science research, that is. Thats why Grossman decided, two years ago, to make the move from the University of California ...

Recommended for you

It sounds like a science-fiction nightmare. But "killer robots" have the likes of British scientist Stephen Hawking and Apple co-founder Steve Wozniak fretting, and warning they could fuel ethnic cleansing and an arms race.

Photos. Messages. Bank account codes. And so much more—sit on a person's mobile device, and the question is, how to secure them without having to depend on lengthy password codes of letters and numbers. Vendors promoting ...

A startup team calls their work a product. They also call it a social movement. Many people in the over-7,000 islands in the Philippines lack access to electricity .The startup would like to make a difference. Their main ...

Do you think that that is why trees are kinda 3d tower shaped thingies? Next they'll discover that the best way to arrange the individual cells is to have them dangling from horizontal posts radiating out from the central mast of the tower...

Any 3D shape is less efficient than simple two-axis heliostat, which always keeps the panel optimally oriented.

Yes, so what? As they mentioned, "solar cells have become less expensive than accompanying support structures, wiring and installation. As the cost of the cells themselves continues to decline more quickly than these other costs, ... the advantages of 3-D systems will grow accordingly." Adding tracking capabilities would greatly increase the expense of the system and the likelihood of it breaking.

I have mention in the past that it might be advantageous to make solar cells(if possible that are transparent that are backed by a reflective material and formed in the same shape as the solar panels used here: http://www.physor...ght.html so that they could reflect un-absorbed light spectrum onto the focal point on the towers giving it two chances to be turn into electricity.

@hemitite Do you think that that is why trees are kinda 3d tower shaped thingies?

Yes trees are fractiline structure which optimize collection of electromagnetic radiation in a fixed spectrum ideal for chlorophyll. Odd that solar technology is employing flat panels when other EM technologies are building advanced fractal antennas, which still fare poorly against nature's multifractal examples.

"Any 3D shape is less efficient than simple two-axis heliostat" I don't agree. If you define efficiency based on land area taken up by the panels. Think of the roof of a building. If you construct a tall structure as illustrated in the article on top of the building - you will be able to harvest a lot more light than just the light hitting the top of the building. You may be stealing your neighbors sunlight - so I guess that could turn into a problem - then we would be suing each other for access to sunlight.

Erm...you can't really cheat the solar constant (i.e. you can't gather up more solar power than hits the Earth per square meter).

Sure those towers get 20 times the energy than just covering the same base area in solar cells BUT you're also creating a shadow that covers at least 20 times that area. Especially when you go to early morning hours or late afternoon the shadow will lengthen and cover a lot more area.

In effect this means: You can either build flat panels much closer together or you can build some towers but with a lot sparser distribution.

Given that it's a lot easier to service and clean stuff that is close to the ground I'm not sure such towers make much sense.

In cities that would only be suitable on rooftops - at the expense of anyone else in the vicinity wanting to gather sunlight.

Yes trees are fractiline structure which optimize collection of electromagnetic radiation in a fixed spectrum ideal for chlorophyll.

Trees have their leaves everywhere because they don't know where the sun is going to be, and where the shadows are going to be, and because they can make use of diffuse ambient light that solar panels can't. They also do turn their leaves towards the sun - if you've ever grown a potted plant, you'd know they do turn towards the brightest light in the room. Sit down and watch, and you can see it.

Plants are, surface area to surface area, much less efficient in collecting solar energy than man made solar panels are, but their advantage is that they collect it consistently and constantly as long as there is any light hitting the leaves from any direction.

That is what this solar tower concept is trying to replicate: use more panels, but point them everywhere to always catch at least some light from somewhere.

Remind me again why we're not building these things in Space, where there's a lot of free, uh, space?

Because you get a LOT more energy out of burning the rocket fuel you'd need to hoist these things up there and turning it into steam/electricity than you would ever get over the lifetime of a space based solar installation?

Remind me again why we're not building these things in Space, where there's a lot of free, uh, space?

Because you get a LOT more energy out of burning the rocket fuel you'd need to hoist these things up there and turning it into steam/electricity than you would ever get over the lifetime of a space based solar installation?

A little research shows thats not true:

""The networked approached really suggests that a breakthrough is possible in terms of schedule, and with the modular program a breakthrough in terms of cost," said Mankins. He suggested that a pilot demonstration could be launched for $10 billion dollars, within ten years, and could generate ten megawatts electricity, comparable to small terrestrial solar plants today."

if this is undeniably true, this is just a compensation for a flawed material. Put simply, in the same 3-D geometry, a Stirling engine would benefit from this research several times more than the accommodated PV panels because the overall efficiency of the device is higher.

Not to mention the fact that the purpose of this research was to lower the cost for people wanting to install solar panels, for the purpose of generating independent energy. If the cost of a Stirling engine is 1/10th of the price of a solar panel array, then those researchers over at MIT are just ignorant for holding on to PV solar panels

$10 billion dollars, within ten years, and could generate ten megawatts electricity, comparable to small terrestrial solar plants today."

You have any idea how many Earth based solar power plants you could build with that? Powerplants that CAN be serviced if need be? That are not open to sudden destruction by solar storms or some rogue country doing a missile test?(Hint: about 2.5GW worth - which is already including salt storage for around the clock electricity production)

Even if the ones on Earth were only to be a third as effective as those in space that's a HELL of a lot better bang for the buck than space based solar.

At the moment the only thing that space based solar has going for it is the "cool-tech" factor. Until we find a REALLY cheap way to put stuff into orbit (like a space elevator) this is a no-go.